In microelectronic applications, polymers that demonstrate high temperature resistance such as polyimides and polybenzoxazoles are generally well known. Precursors of such polymers, can be made photoreactive with suitable additives. The precursors are converted to the desired polymer by known techniques such as exposure to high temperatures. The polymer precursors are used to prepare protective layers, insulating layers, and relief structures of highly heat-resistant polymers.
As the dimension of the photolithographic pattern on a wafer continues to shrink below 0.18 microns, greater demands are placed on lithographic equipment and materials. To meet this challenge, the semiconductor industry is changing from aluminum based alloys and silicon dioxide to copper metal and low dielectric constant (low-k) materials to manufacture chips. When using the new low-k dielectrics there is a decrease in capacitance, which is critical to improving integrated circuit performance, especially for higher density memory chips. Copper is known to have as much as 40% decreased electrical resistance, carries higher current densities, and has improved electromigration resistance compared to aluminum. Thus, copper interconnects allow decreasing transistor size and shorter wire lengths that result in faster, more powerful devices. Fabrication costs are also lower than with aluminum since copper is less expensive and requires fewer processing steps to produce devices.
Copper metallization provides challenges to the coatings industry since copper can act as a catalyst and destabilize systems that are optimized for coating over aluminum. In addition, cuprous and cupric ions present on the copper surface can bind strongly with some polymers and reduce the ability to dissolve off the polymers during certain wafer processes. This results in the presence of some residue on copper surfaces after it is coated by a photosensitive composition, softbaked, exposed and developed. With the increased use of copper metallization in semiconductor devices, it is important to develop coating systems that are compatible with copper and copper processing.
Conventional positive-working photosensitive polybenzoxazole (PBO) compositions contain an alkaline soluble PBO precursor and a diazoquinone photoactive compound (PAC) as disclosed in U.S. Pat. Nos. 4,339,521, 4,371,685, 4,849,051, and 5,376,499. The diazoquinone compound inhibits the solubility of PBO precursor in an aqueous base. However after exposure to light, the diazoquinone compound undergoes photolysis and converts to indene carboxylic acid, which promotes solubility of the PBO precursor in the aqueous base. U.S. Pat. No. 5,037,720 disclosed a positive-working photosensitive polybenzoxazole composition containing an alkaline soluble PBO precursor in which part of OH groups were substituted with a diazoquinone moiety. In this case no PAC was used in the composition. U.S. Pat. Nos. 6,177,225 and 6,214,516 disclosed compositions containing both PBO precursor in which part of the OH groups were substituted by diazoquinone moiety and PAC.